The present invention generally relates to devices and methods for measuring properties of fluids. More particularly, this invention relates to a sensing device equipped with a tube through which a fluid flows for sensing lubricity, viscosity, and other rheological properties of the fluid.
Viscosity and lubricity are important fluid parameters for a variety of fluids, including fuels, lubricants, adhesives, paints, oils, tars, electrophoresis gels, syrups, etc. For example viscosity, which is the internal resistance to flow exhibited by a fluid, is a key fluid parameter for lubricants such as automotive engine oils, whose viscosities change over time to the detriment of the components they lubricate. While oil quality sensors based on measuring the dielectric constant or electrical resistance of a lubricant have been developed and are commercially available, viscosity provides a better indication of the condition of an oil (and other lubricants) and when the oil should be replaced. Lubricity, or the coefficient of friction of a fluid, is often employed to characterize lubricants, fuels, diesel fuel additives, bearings, and load bearing surfaces. As with viscosity, the lubricity of fuels and lubricants often changes over time, such as from contamination from water and particulate matter. Consequently, there has been efforts to develop viscosity and lubricity sensors for fuel and lubricating system applications, including engine oil and fuel systems. For example, lubricity has been measured using slipping disks, bearings, shafts, and balls, which typically involve a long testing process requiring a relatively large amount of sample fluid. Techniques developed to measure viscosity have used capillary force, moving paddles, blades, vibrating tuning forks, and hollow tubes or cantilevers immersed in a fluid. More recently, rheometers and viscometers have been developed with a vibrating micromachined silicon cantilever immersed in the fluid of interest, with the resultant damping of the cantilever vibration being used to indicate viscosity.
Viscosity measuring techniques that rely on a vibrating structure require that the vibrating structure be inserted into the fluid being tested so that the fluid surrounds the structure. In contrast, commonly-assigned U.S. Pat. No. 6,647,778 to Sparks discloses a sensing device capable of sensing the viscosity of a fluid flowing through a microelectromechanical system (MEMS). Sparks' sensing device is used in combination with a micromachined resonating tube, preferably of the type disclosed in commonly-assigned U.S. Pat. No. 6,477,901 to Tadigadapa et al. and adapted for resonant sensing of mass flow and density of a fluid flowing through the tube. One embodiment of Sparks' sensing device incorporates second and third micromachined tubes having bridge portions adapted to deflect in response to a pressure change of the fluid flowing therethrough. Sparks ascertains the viscosity of the fluid flowing through the tubes by comparing the pressures of the fluid within the second and third tubes.
U.S. Pat. No. 7,059,176 to Sparks also discloses a method and device for assessing the viscosity of a fluid. Similar to U.S. Pat. No. 6,647,778 to Sparks, U.S. Pat. No. 7,059,176 utilizes a vibrating tube into which the fluid is introduced, but differs by sensing the influence that the fluid has on the vibrational movement of the tube to assess the viscosity of the fluid. More particularly, U.S. Pat. No. 7,059,176 entails introducing a fluid of interest into a passage within a freestanding portion of a tube, vibrating the freestanding portion of the tube at or near a resonant frequency thereof, sensing movement of the freestanding portion of the tube, and then assessing the viscosity of the fluid by ascertaining the damping effect the fluid has on the vibrational movement of the freestanding portion at or near the resonant frequency. The damping effect can be ascertained in reference to, for example, the quality (Q) factor or peak amplitude of the freestanding portion at the resonant frequency, or an amplitude-versus-.frequency plot of the freestanding portion in the vicinity of the resonant frequency.
Notwithstanding the above advancements, there is an ongoing need for techniques by which viscosity and lubricity can be measured, particularly more quickly and using smaller sample sizes than possible with existing lubricity measurement techniques.